Tunnelling spectroscopy is a sensitive technique for measuring the vibrational spectra of molecules, and is a powerful way of identifying molecules and molecular fragments. Since a functional group, e.g. --CH.sub.3, has roughly the same vibrational frequencies wherever it appears in a molecule, researchers can deduce the presence or absence of --CH.sub.3 by the presence or absence of vibrations at its characteristic frequencies. From this knowledge of the presence or absence of functional groups, together with whatever other information is available, researchers can guess the structure of their unknown molecules. After they guess, they can use vibrational spectroscopy to see if they were right or wrong. They do this by comparing the vibrational spectrum of their unknown molecule to the vibrational spectrum of what they guess it is. There are many excellent, extensive collections of vibrational spectra for use in this comparison. If the spectrum of the guessed molecule is not in an accessible collection, the researcher consult the literature or measure it himself.
Tunnelling spectroscopy has a unique combination of spectral range, sensitivity, resolution, and selection rules. It is especially suited to the vibrational spectroscopy of adsorbed monolayers on surfaces.
FIG. 1 shows an idealized view of a tunnel junction. In this junction, current is measured as a function of voltage. Two components are found: a steadily increasing current due to elastic electron tunnelling and a current, which has a threshold voltage h.nu./e and increases steadily thereafter, due to inelastic electron tunnelling. This threshold is set by the requirement that electrons must give up an energy h.nu. to excite the molecular vibration. Since their tunnelling energy is eV, the requirement exists that eV is approximately greater than or equal to h.nu..
FIGS. 2a-c show the total current I, which is the sum of the current through the elastic and inelastic tunnelling channels. It has a kink at V=h.nu./e (FIG. 2a), which becomes a step in dI/dV versus V (FIG. 2b), and a peak in d.sup.2 I/dV.sup.2 (FIG. 2c). A plot of d.sup.2 I/dV.sup.2 versus V is referred to as a tunnelling spectrum.
Accordingly, a tunnelling spectrum reveals the vibrational energies of molecules included between two electrodes, since a vibrational energy of h.nu. results in a peak at V=h.nu./e. A real tunnelling spectrum has many peaks since the typical molecules that are studied have many vibrational modes.
The most serious limitation of tunnelling spectroscopy is that it can only be performed with tunnel junctions. A typical prior art "oxide barrier" tunnel junction is shown in FIG. 3. Such a junction consists of a rigid substrate 10 having evaporated thereon a bottom electrode 12, typically of aluminum. Bottom electrode 12 is then oxidized and doped in a manner to obtain a monolayer of chemisorbed molecules, which are the molecules to be studied. Last, a top electrode, typically lead, is evaporated over the bottom electrode 12 and onto the substrate, so as to "sandwich" the doped molecules between the two electrodes. The vibrational spectra is then obtained.
The prior art "oxide barrier" junctions discussed above suffer from several inherent limitations. First, the bottom electrode can only be a metal that forms a thin, pinhole-free oxide of high dielectric strength. Second, the junction cannot, in general, be adjusted after it is made. Third, for tunnelling spectroscopy, the unknown must be sandwiched between the oxide and the top electrode.
It has also been recognized that, to be able to measure a tunnelling current, the electrodes must be spaced no more than 100 angstroms apart, which precluded the use, in the prior art oxide barrier junctions of an air or a vacuum between the electrodes due to problems of vibration. No prior art junction has disclosed a mechanically-adjusted metal-insulator-metal junction. Such a junction would overcome all of the limitations mentioned above for oxide barrier junctions.
Accordingly, it is the principal object of the present invention to mechanically adjust an artificial barrier junction.
Another object of the present invention is to utilize a mechanically adjustable artificial barrier junction with a vacuum, gas, or liquid.